LetEncrypt does validate DNSSEC signatures (when they exist), but CA's aren't even required to do that. LetsEncrypt also does multi-perspective lookups, so a single hijacked DNS transaction or poisoned cache is insufficient to trick it. Hopefully, at some point in the not-too-distant future, LE's multi-perspective lookup will generate data we can look at about the frequency of DNS attacks on certificate issuance. I expect it to be quite rare, because it's an elaborate attack with a weak payoff.

The right way to think about this CA issue is this:

* The largest, best-funded, savviest security teams in tech are, like the rest of tech, not signing their domains; the major TLDs are overwhelmingly not signed (there is low single digit uptake in .COM for instance, and what's there is overwhelmingly not big companies but rather random domains signed by registrars that auto-sign). Nobody who's actually targeted for CA misissuance attacks uses DNSSEC to mitigate that threat.

* The WebPKI already has a system in place to guard against misissuance that, unlike DNSSEC, actually does work: Certificate Transparency. So if you're actually concerned about CAs not issuing bogus certs for you, match your revealed preferences to your stated ones and set up CT monitoring.